1. Field of the Invention
The present invention relates to a pulse-type gas concentration measurement system and a method for pulse-type concentration measurement for volatile chemical matter in a specific environment.
2. Brief Discussion of the Related Art
Conventionally, a gas concentration sensor is used for obtaining the concentration of a specific gas. An example of a conventional gas concentration sensor 500 is described in detail with reference to FIG. 1A and FIG. 1B.
The conventional gas concentration sensor 500, as shown in FIG. 1A, has a body 510, voltage input elements 520, and output elements 530. The body 510, as shown in FIG. 1B, has a substrate 512, electrodes 514, a sensing element 516, and a heater 518. Generally, the sensing element 516 is a metallic oxide membrane, such as a tin dioxide (SnO2) membrane, which reacts to a specific gas in the vicinity of the gas concentration sensor 500. When the conventional gas concentration sensor 500 is applied in a specific environment to measure gas concentration, a fixed voltage is input to the sensor 500 through the voltage input element 520 to activate the heater 518, to heat the membrane of the sensing element 516 to a predetermined temperature, such as 400° C. Thus, the membrane of the sensing element 516 reacts to the specific gas to be measured in the specific environment, and the resistance of the sensing element 516 changes due to the reaction. An outgoing voltage, determined by the resistance of the sensing element 516, is then obtained by the output element 530 as an outgoing signal.
It is obvious that the concentration of the specific gas in the specific environment affects the reaction, and the relation between the concentration of the specific gas and the resistance of the sensing element 516 can be established by experiment as a reference for the gas concentration sensor 500.
FIG. 2A is a chart showing an example of gas concentration measurement using the conventional gas concentration sensor 500, in which the curves L1 and L2 respectively refer to different concentrations of the specific gas. When the voltage is input to the sensor 500 through the voltage input element 520 to activate the heater 518, the membrane of the sensing element 516 is heated to a predetermined temperature, such as 400° C. In both cases, the resistance of the sensing element 516 changes due to the reaction, inducing an outgoing voltage (shown as point A) with concentration L1 and point B with concentration L2. It should be noted that the predetermined temperature of the conventional gas concentration sensor 500 is generally set to a preferred temperature, in which the outgoing voltage is significant, so that responses of the gas concentration sensor 500 are obvious. Fox example, the preferred temperature shown in FIG. 2A is approximately 400° C.
The conventional gas concentration sensor 500 has a membrane-type structure, which has a relatively low cost. Further, the conventional gas concentration sensor 500 reacts to the gas to be measured rapidly and can be used effectively for a long period of time. As a result, the gas concentration sensor is widely used. For example, U.S. Pat. No. 6,336,354 discloses a gas concentration measuring apparatus, in which a gas concentration sensor is applied, using a heat control circuit to supply power to the heater of the sensor cyclically using a pulse-modulated (PM) signal. In this case, the apparatus corrects errors contained in the gas concentration signal, regulating the signal, and the outgoing signal of the gas concentration sensor is significant.
The conventional gas concentration sensor 500, however, is used mainly to measure the concentration of a specific gas. It is obvious that the conventional gas concentration sensor 500 can be used in a specific environment when the specific gas exists in the specific environment. The membrane of gas concentration sensor 500, however, may react to a plurality of gases. Thus, when more than one of the gases exists in the specific environment, the conventional gas concentration sensor 500 cannot distinguish between each gas, so that the outgoing signal of the gas concentration sensor 500 does not correspond exactly to a specific gas, and gas concentrations are not accurately obtained. Additionally, when the composition of the gas in the specific environment is unidentified, the conventional gas concentration sensor 500 cannot determine the composition of the gas.
In a gas concentration measurement, a fixed voltage is input to a sensor to activate a heater, thus heating the membrane of the sensing element to react with the specific gas to be measured in the specific environment, and the resistance of the sensing element changes due to the reaction. An outgoing signal is thus obtained. The fixed voltage is generally set to heat the membrane of the sensing element to a preferred temperature. However, if the voltage input to the sensor is changed, the outgoing signal also changes.
FIG. 2B is a chart showing outgoing signals corresponding to a plurality of gases measured by a sensor. The gases include hydrogen (H2), carbon monoxide (CO), ethanol (C2H5OH), methane (CH4) and butane (C4H10), and the concentration of each gas is kept at 0.1% to obtain the outgoing signals. The sensor applied is a widely-used conventional gas concentration sensor as described above. It should be noted that the curves of FIG. 2B show that the outgoing signals of the gases change corresponding to the membrane temperature (that is, the voltage input to the sensor), and each outgoing signal can be recognized as a distinctive pattern. Thus, the outgoing signal of the gas can be used as a chemical matter characteristics signal of the gas for use in gas identification.
The gas identification method in FIG. 2B can be further described in comparison to the gas concentration measurement, as shown in FIG. 2A. In FIG. 2A, the outgoing signal of the gas concentration sensor 500 is a point, related to a fixed membrane temperature (due to the fixed voltage input to the sensor 500), such as the preferred temperature. However, in FIG. 2B, the outgoing signal of the sensor is a curve related to a specific range of membrane temperature, applied as determined by the chemical matter characteristics signal of the gas in association the inventor has proposed an intelligent gas identification system and method thereof, which is disclosed in Taiwan Patent No. 531139. In the intelligent gas identification system and method thereof, a pulse-modulated (PM) signal is used as the input voltage to the conventional gas concentration sensor so that the outgoing signals corresponding to various gases differ. Thus, a chemical matter characteristics database can be established by experiment, and the chemical matter characteristics can be used as a reference to determine the composition and/or concentration of the gases.
Specifically, Taiwan Patent No. 531139 utilizes the gas identification method shown in FIG. 2B, which can be further described with respect to FIG. 3A and FIG. 3B, also of this disclosure.
FIG. 3A shows the intelligent gas identification system disclosed in Taiwan Patent No. 531139. The intelligent gas identification system is applied to perform gas identification (or volatile chemical matter identification) in a specific environment, which has a sensor 10, a pulse power supply module 20, and a processing device 30.
The sensor 10, which can be a conventional gas concentration sensor 500 as shown in FIG. 1A, has at least a voltage input element, at least an output element, and a sensing element (that is, the body 510). The sensing element can be a metallic oxide membrane, such as a tin dioxide membrane (SnO2), which reacts to the specific gas in the vicinity of the sensor 10.
The pulse power supply module 20 is connected to the voltage input element of the sensor 10 to send a variable pulse-modulated voltage to the sensor 10, so that the sensor 10 sends out an outgoing signal through the output element.
The processing device 30 can be a computer with a pattern recognition module and a database for storing a plurality of chemical matter characteristics signals. The pattern recognition module, for example, can be graphic recognition software. Further, the processing device 30 receives an outgoing signal from the output element of the sensor 10.
When the intelligent gas identification system is used to perform gas identification, the sensor 10 is disposed in the specific environment. The pulse power supply module 20 sends a variable pulse-modulated voltage to the sensor 10 through the voltage input element, so that the membrane of the sensing element is reiteratively heated, and in each heating process, the membrane temperature varies due to the variable pulse-modulated voltage. Thus, the membrane reacts to the gas in the specific environment with different temperature, and the sensor 10 sends out an outgoing signal, such as a variable pulse-modulated signal, to the processing device 30. Then, the processing device 30 compares the outgoing signal with the chemical matter characteristics signals to determine an identification result for the gas, such as composition of the gas, and/or concentration of the respective constituents of the gas.
The method of gas identification disclosed in Taiwan Patent No. 531139 can be described with reference to the flowchart of FIG. 3B. According to FIG. 3B, it is assumed that a gas G to be identified exists in a specific environment, and two given chemicals X and Y are provided for comparison to the gas G. That is, the gas identification is performed to determine if the gas G matches X or Y exactly.
When gas identification is performed, a sensor 10 as mentioned is provided (step S10) and disposed in the given chemicals X and Y (step S20). Then, a variable pulse-modulated voltage is provided to the sensor 10 respectively, so that the sensor 10 outputs the chemical matter characteristics signals SX, SY for the given chemicals X and Y (step S30). The chemical matter characteristics signals SX and SY can then be stored in a database (step S40) for further identification of the gas G.
The sensor is then disposed in the specific environment with the gas G (step S50). The sensor is provided with a variable pulse-modulated voltage, so that the sensor outputs an outgoing signal SG corresponding to the gas G in the specific environment (step S60). Thus, the processing device 30 receives the outgoing signal SG and compares the outgoing signal SG to the chemical matter characteristics signals SX and SY to determine an identification result for the gas G (step S70).
Preferably, the intelligent gas identification system and method thereof is utilized in a specific environment, in which the gas G to be identified is unknown. For example, the intelligent gas identification system is suited to a semi-open environment, in which composition of the gas G is variable.
In some cases, however, the specific environment is an airtight environment that has a fixed composition of the gas G, or an environment in which the composition of the gas G is already known. For example, in a laboratory or a semiconductor fab, it is well known that a chemical reaction or a manufacturing process may produce certain types of gases. Thus, composition of the gas in the laboratory or the semiconductor fab is fixed in a group of these types of gases. It should be noted that the above-mentioned process of receiving the outgoing signal SG and comparing the outgoing signal to the chemical matter characteristics signals SX and SY can be performed only once to determine or confirm the composition of the gas G. Accordingly, the intelligent gas identification system and method is complicated and can be further simplified.
Further, the sensitivity of the sensor 10 can be strongly influenced by environmental factors, such as temperature or air flow rate, in which temperature is the most critical factor. Accordingly, a low-temperature environment has a destabilizing effect on the relationship between the outgoing voltage and the input voltage, resulting in inaccurate gas identification.